Biogeosciences, 8, 415–431, 2011
www.biogeosciences.net/8/415/2011/
doi:10.5194/bg-8-415-2011
© Author(s) 2011. CC Attribution 3.0 License.
Biogeosciences
Productivity patterns and N-fixation associated with
Pliocene-Holocene sapropels: paleoceanographic and
paleoecological significance
D. Gallego-Torres1,5 , F. Martinez-Ruiz2 , P. A. Meyers3 , A. Paytan4 , F. J. Jimenez-Espejo2 , and M. Ortega-Huertas5
1 Utrecht
University Faculty of Geosciences (Department of Earth Sciences – Geochemistry) Budapestlaan, 4, 3508 TA
Utrecht, The Netherlands
2 Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR) Facultad de Ciencias, Campus Fuentenueva, 18002 Granada, Spain
3 Department of Geological Sciences, The University of Michigan, Ann Arbor, MI 48109-1005, USA
4 Institute of Marine Sciences, Earth & Planetary Sciences Department, Univ. of Santa Cruz Santa Cruz, CA 95064, USA
5 Departamento de Mineralogı́a y Petrologı́a, Facultad de Ciencias, Universidad de Granada, Campus Fuentenueva, 18002
Granada, Spain
Received: 17 May 2010 – Published in Biogeosciences Discuss.: 11 June 2010
Revised: 25 November 2010 – Accepted: 31 January 2011 – Published: 16 February 2011
Abstract. We have studied a suite of 35 sapropel sequences
from a transect of four ODP sites across the Eastern Mediterranean to explore for paleoproductivity patterns and provide new insights on ecological changes during their deposition. Paleoproductivity variations were identified using TOC
and Babio mass accumulation rates and δ 15 Ntotal and δ 13 Corg
values. Elevated Ba/Al and TOC mass accumulation rates
record periods of basin-wide amplified productivity. Our
data further support that sapropels were formed by cyclic
increases in primary production of marine organic matter
largely sustained by N-fixing bacteria. This productivity increase was triggered by climate factors leading to increased
fluvial discharge and amplified nutrient input that also favored the establishment of N-fixing bacteria. Enhanced productivity led to depletion of deepwater dissolved oxygen and
consequently improved organic matter preservation. Primary
production was more intense during the middle to Late Pleistocene compared to Pliocene equivalents, coinciding with increasing total sedimentation rates. δ 15 N values are dramatically lower in the sapropels than in TOC-poor background
sediments, indicating a major contribution from nitrogenfixing bacteria to the higher productivity during sapropel deposition. Additionally, different degrees of denitrification occurred as a consequence of water column oxygenation which
in turns evolved from stagnant anoxic bottom waters during
Pliocene sapropels to oxygen depleted and sluggish circu-
Correspondence to: D. Gallego-Torres
([email protected])
lation in late Quaternary layers. These differences between
sapropel layers provide new evidences for the general evolution of the Eastern Mediterranean basin during the last 3 Mys
in terms of paleoceanographic conditions and the intensity of
climate variability leading to sapropel deposition.
1
Introduction
The multiple sapropel layers that were deposited in the
Neogene-Quaternary Mediterranean basin are of particular
interest to the topic of enhanced organic carbon accumulation in marine sediment. Sapropels, as defined by Kidd
(1978), are cyclically deposited dark colored sediment layers that are more than 2 cm in thickness and contain more
than 2% total organic carbon (TOC). A debate about whether
improved preservation or elevated productivity is the major
cause for high concentrations of organic matter in such sediments has persisted in paleoceanographic studies for decades
(e.g., Casford et al., 2003; de Lange et al., 2008; Emeis and
Weissert, 2009; Jenkins and Williams, 1984; Mangini and
Schlosser, 1986). The traditional interpretation of sedimentary layers rich in organic matter as representative of anoxic
events has been challenged by Pedersen and Calver, (1990;
see also Calvert et al., 1996; Calvert and Pedersen, 1993),
who argued that increased productivity, rather than absence
of oxygen, was the primary factor responsible for the enhanced accumulation of organic carbon in sediments. Indeed, sapropel deposition is usually considered to be related
to an increase in marine export productivity (e.g., Calvert et
Published by Copernicus Publications on behalf of the European Geosciences Union.
416
D. Gallego-Torres et al.: Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels
al., 1992; Diester-Haass et al., 1998; Lourens et al., 1992;
Martı́nez-Ruiz et al., 2000, 2003; Meyers and Arnaboldi,
2005; Weldeab et al., 2003a, b). However, how marine
productivity increased in a presently oligotrophic, nutrient
deficient basin like the Mediterranean raises questions regarding the sources of nutrient that have fueled this productivity increase (e.g., Casford et al., 2003; Filippelli et
al., 2003; Menzel et al., 2003; Sachs and Repeta, 1999;
Tanaka et al., 2010).
Development of a number of geochemical paleoproductivity proxies that are independent of oxygen abundance has
allowed reconstruction of productivity and improved interpretation of the causal mechanisms of organic matter accumulation. Among these proxies, barium excess (Babio ), and
biogenic barite accumulation (e.g., Dehairs et al., 1987; Dymond and Collier, 1996; Dymond et al., 1992; McManus et
al., 1998; Paytan and Kastner, 1996), and nitrogen content
and isotopic composition (e.g., Altabet and Francois, 1994;
Arnaboldi and Meyers, 2006; Calvert et al., 1992; Meyers
and Bernasconi, 2005; Peters et al., 1978) have been found
especially useful in a variety of paleoceanographic settings.
In recent years several papers have employed some of these
paleoproductivity proxies to describe spatial and temporal
patterns of sapropel deposition (e.g., Arnaboldi and Meyers, 2006; Meyers and Bernasconi, 2005; Rinna et al., 2002;
Struck et al., 2001). However, further comparisons of the
isotopic and elemental proxies and Babio , paleoproductivity
proxy most widely used in the Mediterranean, and additional
documentation of any differences of these signals during the
paleoceanographic evolution of the basin are important to resolve existing questions about the conditions under which
sapropels were deposited.
In this paper, we describe the results of our analyses of a
suite of 35 sapropels of different ages that span the Pliocene
through the Holocene and that originate from a four-site transect across the Eastern Mediterranean Basin. The description
of the spatial and temporal patterns of export productivity in
sapropel deposition is interpreted in terms of their possible
paleoceanographic significance. We correlate both organic C
and total N content and isotope signatures (δ 15 N and δ 13 C)
to complement the information given by Ba/Al, and we employ TOC and Babio mass accumulation rates (MAR’s) to
refine paleoproductivity patterns. The effect of diagenesis on
sediment N and C contents is also considered, inasmuch as
organic matter is susceptible to post-depositional oxidation
and alteration (e.g., Martı́nez-Ruiz et al., 2000; Thomson et
al., 1995; Van Santvoort et al., 1996).
2
2.1
Materials and methods
Sample settings
Fig. 1: Location map for the 4 studied sites. Site 964 (Ionian basin) is the deepest studied
Fig. 1. 966
Location
map
thehigh,
4 studied
sites. Site
964 (lonian
location.
is located
on a for
pelagic
the Erathostenes
Seamount
(see textbasin)
for
details).
is the deepest studied location. 966 is located on a pelagic high, the
Erathostenes Seamount (see text for details).
summarize, the sedimentary sequence deposited at Site 964,
located in a deep marine setting (3658 m.b.s.l.) on the Pisano
Plateau (Ionian Basin), is influenced by the Adriatic Sea
and the water masses coming from the Western Mediterranean basin through the Strait of Sicily. Site 969, also in
a relatively deep, open marine setting (2200 m.b.s.l.) on the
Mediterranean Ridge, represents the centermost location in
the Eastern Mediterranean. Cores recovered at Site 967 in
the Levantine Basin, although containing a sequence of deep
pelagic sediments (2555 m.b.s.l.), are influenced by detrital
input from the Nile River, which drains the central African
craton. Finally, Site 966 is situated on a pelagic high, the
Eratosthenes Seamount, at the relatively shallow water depth
of 926 m.b.s.l. At each site, high resolution sampling was
carried out on selected sapropel-containing core intervals.
These sections represent late Pleistocene and Holocene periods of sapropel deposition at all four sites, Lower Pleistocene
layers at sites 969 and 967 and Pliocene layers for sites 964
and 969.
The sediments in these cores are composed mostly of
variably bioturbated nannofossil clay, clayey nannofossil
ooze and nannofossil ooze with some intervals of clay and
foraminifera sand, variably bioturbated (Emeis et al., 1996).
Dark colored to black sapropel layers appear periodically
throughout the pelagic sequences. Some of these TOCenriched sediments and sections of the overlying and underlying sediment from these cores were sampled at 2 cm intervals. Where lamination allowed, samples were collected at a
resolution as fine as 1 mm.
Sapropel sequences were sampled in cores recovered at four
ODP Leg 160 sites (Fig. 1) that represent different oceanographic regimes within the Eastern Mediterranean Sea. To
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D. Gallego-Torres et al.: Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels
2.2
Analysis
Samples were dried, homogenized in an agate mortar, and
then subdivided for different analyses. TOC measurements
for some samples were carried out using a Perkin-Elmer Elemental analyzer at the Stable Isotope Laboratory at Stanford University (Mucciarone, 2003). The rest of the samples were analyzed for total carbon (TC) and TOC separately
at Bremen University using a TOC/TC analyzer. TOC was
measured on the TOC/TC analyzer after acidification of sediment with 1N HCl to remove carbonates followed by heating
to dryness. At Stanford University, parallel to TOC measurements, total N (TN), δ 13 C and δ 15 N isotopic composition were also carried out using a Finnigan MAT isotope
ratio mass spectrometer (IRMS) connected to a Carlo Erba
(now CE Elantech, Inc.) NA1500 Series II Elemental Analyzer. TOC/TN ratios are expressed on an atom/atom basis. δ 15 N and δ 13 C values are respectively expressed relative to atmospheric dinitrogen and Vienna PeeDee Belemnite
(VPDB) standards. These samples were repeatedly acidified
with HSO3 to eliminate all inorganic carbon prior to isotope
analysis.
Barium content was determined using an ICP-MS PerkinElmer Sciex Elan 5000 spectrometer (CIC; Analytical Facilities of the University of Granada), using Re and Rh as internal standards. Coefficients of variation calculated by dissolution and subsequent analyses of 10 replicates of powdered
samples were better than 3% and 8% for analyte concentrations of 50 and 5 ppm, respectively (Bea, 1996). Aluminium
content was analyzed by atomic absorption spectrometry at
the Analytical Facilities of the University of Granada. These
two analyses were carried out after HNO3 and HF total digestion of the homogenized sample. Biogenic Ba (Babio )
was calculated following the equation used by Wehausen and
Brumsack (1999) in order to separate detrital barite from barium precipitated as biogenic barite.
Ages of individual sapropel layers and site to site age correlations are based on precessional age scales in Emeis et
al. (2000a), Lourens (2004), and Sakamoto et al. (1998).
Each sampled layer was directly assigned to the corresponding sapropel name and insolation cycle (i-cycle) using Tables 1 to 4 from Emeis et al. (2000a) on the basis of sample to sample correlation (using ODP nomenclature) and/or
revised composite core depths, or correlated using core tie
points obtained from the ODP online database.
TOC and Babio MAR’s were calculated based on the
sediment dry bulk density (DBD) obtained from the ODP
Leg 160 database (available online) and linear sedimentation
rates (LSR), calculated from age models from the database:
MAR = DBD*LSR. When borehole LSR were not available,
they were calculated for our sampling intervals, either using
adjacent age intervals (again obtained from the ODP online
database) or based on the methodology used by Meyers and
Arnaboldi (2005). In brief, the peak TOC MAR calculated
in each sapropel layer was assumed to represent the orbitally
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417
Table 1. Summary of analyzed sapropel layers with their correspondent depth, assigned insolation cycle, age (mid-point) and calculated Linear Sedimentation Rate (LSR) for each interval.
Sample Location
I-Cycle
*Age sapropel
midpoint (ka)
Depth
(mbsf)
Depth
(mcd)
LSR
(cm/ky)
0.73
7.71
11.12
11.99
12.62
76.73
79.64
80.19
80.93
0.73
7.81
11.22
12.09
12.72
98.21
101.12
101.67
102.41
4.14
5.93
6.00
4.04
3.02
2.55
2.41
2.87
2.50
0.27
2.36
3.31
4.36
4.78
5.45
0.27
2.36
3.31
4.36
4.78
5.45
3.90
2.60
2.35
2.19
2.24
2.04
1.17
4.32
5.65
10.57
47.76
48.21
49.35
49.89
50.35
50.87
1.17
4.28
5.58
9.79
52.96
53.41
54.55
55.09
55.55
56.07
5.36
5.64
5.90
4.67
2.20
2.58
1.92
2.42
2.30
2.18
0.27
2.64
3.92
5.65
6.16
27.05
28.54
50.74
51.48
0.27
2.42
3.41
4.94
5.48
34.05
36.19
73.25
73.96
4.39
4.36
4.24
2.62
2.64
2.80
2.75
1.88
2.37
Site 964, Ionian Basin (3658 m.b.s.l.)
Hole 964A
Hole 964A
Hole 964A
Hole 964A
Hole 964A
Hole 964A
Hole 964A
Hole 964A
Hole 964A
i-c 2
i-c 12
i-c 16
i-c 18
i-c 20
i-c 272
i-c 282
i-c 284
i-c 286
7.75
124
172
195
217
2828
2943
2965
2989
Site 966. Eratosthenes Seamount (926 m.b.s.l.)
Hole 966B
Hole 966B
Hole 966B
Hole 966B
Hole 966B
Hole 966B
i-c 2
i-c 8
i-c 12
i-c 16
i-c 18
i-c 20
8.26
81
124
172
195
217
Site 967. Levantine Basin (2555 m.b.s.l.)
Hole 967D
Hole 967D
Hole 967D
Hole 967D
Hole 967D
Hole 967C
Hole 967C
Hole 967C
Hole 967C
Hole 967C
i-c 2
i-c 8
i-c 10
i-c 16
i-c 170
i-c 172
i-c 176
i-c 178
i-c 180
i-c 182
7.94
81
102
172
1736
1757
1808
1829
1851
1872
Site 969. Mediterranean Ridge (2200 m.b.s.l.)
Hole 969A
Hole 969A
Hole 969A
Hole 969A
Hole 969A
Hole 969A
Hole 969A
Hole 969A
Hole 969E
i-c 2
i-c 10
i-c 12
i-c 16
i-c 18
i-c 152
i-c 160
i-c 280
i-c 284
7.98
102
124
172
195
1564
1642
2921
2965
*Ages are from Lourens et al. (1996) and de Kaenel et al. (1999)
and are based on the orbital calculations. S1 age is based on De
Lange et al. (2008).
tuned age of the corresponding i-cycle. For sapropel 1, the
age model obtained by de Lange et al. (2008) was applied,
whereas for each of the rest of the analyzed sapropels, its
corresponding i-cycle was assigned following the nomenclature and ages defined by Emeis et al. (2000a). The difference in core depths between successive sapropel layers was
then divided by the ∼21 ky of each precessional cycle to arrive at a linear sedimentation rate for this time and place.
This method is consistent with the age model published by
Lourens (2004) although it may sometimes contradict the
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418
D. Gallego-Torres et al.: Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels
assumptions of sapropel synchronicity considered by other
authors (e.g., Cane et al., 2002; Capotondi et al., 2006; de
Lange et al., 2008; Emeis et al., 2003). The LSRs used for
MAR calculations are summarized in Table 1.
3
3.1
Results
TOC concentrations and MAR’s
TOC concentrations in the samples range from less than
0.1% in the background sediment up to 28.4% in one sapropel layer (Figs. 2 to 6), with the highest values detected in the
Pliocene sapropels at sites 964 and 969. The general temporal trend is a progressive increase in sapropel TOC concentrations from the Holocene through the Late Pleistocene
(Figs. 2 to 5) and into the Early Pleistocene and Pliocene
(Fig. 6). At each site, TOC-MARs show maximum values in
Pliocene sapropels. For sapropels deposited during the same
insolation cycle, those at Site 966 have the lowest TOC concentrations and lowest TOC MARs (see Fig. 2 to 5).
3.2
Organic carbon isotopic compositions
The Corg -isotopic compositions of the sapropel sequences
differ spatially and temporally (see Figs. 7 to 11). As a
whole, δ 13 C values range between −19.0‰ and −26.5‰,
and both the highest and lowest values are found within the
Late Pleistocene sapropels sampled from Site 967. Although
a general trend of increasing δ 13 C values appears at sites 964
and 966, this tendency is not observed for Site 967 nor in the
Pliocene S53 at Site 969. An apparent shift towards higher
C-isotopic compositions occurs at the base of the Quaternary
sapropels or just before the increase in their TOC content.
3.3
Nitrogen isotopic compositions
The δ 15 N values are strikingly similar at all four locations.
They are significantly lower in the TOC rich sections compared to the carbonate oozes. The δ 15 N values range from
≈1.0 to 0.0 for the sapropel 1 (i-cycle 2) (Fig. 7) to between
0‰ to −3.0‰ for Pleistocene sapropels (see Figs. 8 to 10)
as low as −3.1‰ in the Pliocene sapropels (Fig. 11). When
same i-cycles are compared between sites, δ 15 N values are
very similar; only those values at Site 966 tend to be slightly
higher.
3.4
Ba concentrations and MAR’s
Barium concentrations vary in parallel with TOC concentrations (Figs. 2 to 5), although elevated Ba concentrations
frequently extend above the organic enriched layer in late
Quaternary sapropels (Fig. 2), a feature especially visible
in sapropels from Site 966. The sapropel layers at this site
show a distinct offset between organic carbon accumulation
Biogeosciences, 8, 415–431, 2011
and the productivity signal as represented by Ba/Al. In contrast to the patterns common to the late Quaternary sapropels,
Babio values decrease more than TOC MARs near the tops
of Pliocene sapropels at sites 964 and 969 (Figs. 6 and 11).
Because of the different patterns in the two proxies, we estimated durations of the sapropel formation events by measuring the Ba peak width in the Holocene and Late Pleistocene
sapropels and the TOC MAR peak in the Early Pleistocene
and Pliocene sapropels.
Babio -MARs are largest in Late Pleistocene sapropels, particularly during i-cycle 12 (S5, see Fig. 3). The highest Babio MARs values are found at Site 964, the deepest studied site.
For same-age sapropels, the increases in Babio -MAR are similar for the three sites located in the deeper parts of the basin,
but they are notably lower at shallower Site 966 (Figs. 2 to 5).
3.5
TOC/TN ratios
The atomic TOC/TN ratio varies similarly through time at
the four locations, showing values mostly between 5 and 10
for background “normal” pelagic sediment (carbonate ooze),
and increasing up to 25 in the sapropels (see Figs. 6 to 11).
The highest sapropel TOC/TN ratios are found at Site 967,
whereas the lowest are found at the bathymetrically elevated
Site 966.
4
4.1
Discussion
Paleoproductivity proxies
4.1.1 δ 13 C patterns and paleoceanographic
significance
Organic δ 13 C values in analyzed sediment samples mostly
correspond to organic matter of marine origin, although some
terrestrial influence might be argued. However, δ 13 C values
lower than −23‰ are common across the Eastern Mediterranean, especially in low TOC sections. The more negative
values are consistent with an oligotrophic setting in which C
availability does not limit algal vital fractionation. In contrast, periods of higher productivity result in depletion of
12 C from increased uptake, increasing δ 13 C
org values (e.g.,
Meyers, 1997). This slight increase in δ 13 Corg values is the
general response found in analyzed sapropels and is particularly evident at the base of the Corg -rich layers. This classic
high-productivity isotopic trend is not evident at Site 967.
Instead, cores recovered at this deep Levantine sub-basin location exhibit generally lower δ 13 Corg values and the most erratic trends in the sample suite. A possible explanation is that
extensive organic matter recycling in the surface ocean limited the 13 C enrichment associated with net uptake (e.g., Arnaboldi and Meyers, 2006; Menzel et al., 2003; Meyers and
Arnaboldi, 2008; Struck et al., 2001) which can also apply
to slight decreases in the upper part of some sapropel layers
(e.g., S49, Site 969). Increased continental runoff associated
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D. Gallego-Torres et al.: Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels
964
969
967
966
964
200
967
966
Babio MAR
(ug/cm2/ky)
Ba/Al (X10 -4)
0
969
419
400 0
200
400 0
200
400
0
200
400
0
0
5
5
5
10 0
5
10 0
5
10 0
10
20
15
30 0
15
30 0
15
30 0
15
30
Age (ky)
0
10
10
15
15
0,0
2,0
4,0 0,0
2,0
4,0 0,0
2,0
4,0 0,0
2,0
4,0
0
TOC-MAR (g/cm2/ky)
TOC (%)
Fig. 2. Comparison of TOC and Ba/Al vs. TOC-MAR and Ba bio MAR for S1 on the four studied sites.
shade
to visible sapropel
Light shade represents oxydized sapropel.
Fig. 2. Comparison ofDark
TOC
andcorresponds
Ba/Al vs. TOC-MAR
and Balayer.
bio MAR for S1 on the four studied sites. Dark shade corresponds to visible
sapropel layer. Light shade represents oxydized sapropel.
964
969
966
964
0
250
969
966
Babio MAR
(ug/cm2/ky)
Ba/Al (X10 -4)
500 0
250
500
0
250
500
0
115
120
120
Age (ky)
115
10
20 0
10
20 0
10
20
25
50 0
25
50
1445
25.8
72.1
125
125
130
130
135
135
0
5
10 0
5
10
0
5
TOC (%)
10
0
25
50 0
TOC-MAR (g/cm2/ky)
Fig. 3. Comparison of TOC and Ba/Al vs. TOC-MAR and Ba bio MAR for S5 on three studied sites.
Dark
shade
corresponds
to visible and
sapropel
layer. for
Light
represents
oxydized
sapropel.
Fig. 3. Comparison
of TOC
and
Ba/Al vs. TOC-MAR
Babio MAR
S5 shade
on three
studied sites.
Dark shade
corresponds to visible
Maximum values off-scale are numerically indicated on the graph.
sapropel layer. Light shade represents oxydized sapropel. Maximum values off-scale are numerically indicated on the graph.
with periods of sapropel formation (e.g., Gallego-Torres et
al., 2010; Rossignol-Strick, 1985) lowered the salinity of the
surface ocean and created a strongly stratified water column
that impeded sinking of organic matter and discouraged vertical mixing. Oxidation of the isotopically light organic carbon could then occur within the lower part of the photic zone,
where it could be re-assimilated by photosynthesizers. The
near-surface recycling of marine organic carbon during times
of fluvial dilution of the surface ocean would potentially be
augmented by delivery of low 13 C terrestrial organic matter,
this site being under the influence of the distal plume of the
Nile River, adding to the low isotope signal. In any case,
a shift toward higher isotopic composition is visible at the
base of the TOC-enriched layers at Site 967, which is a clas-
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sical indication of a general increase in 12 C removal in the
form of marine organic matter during initiation of sapropel
deposition.
4.1.2
Paleoproductivity evidence from nitrogen
concentrations
Total nitrogen in sediment indicates organic matter production, provided N adsorbed to clay minerals is insignificant
(e.g., Calvert, 2004; de Lange, 1992; Freudenthal et al.,
2001). This was verified using the correlation method described by Nijenhuis and de Lange (2000), Calvert (2004),
and Arnaboldi and Meyers (2006) in which the concentrations of TOC are plotted against those of TN. The correlation
is extremely good (R > 0.9 for all cores, see Fig. 12) with an
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420
D. Gallego-Torres et al.: Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels
964
969
967
964
966
0
500 0
250
969
967
966
Babio MAR
(ug/cm2/ky)
Ba/Al (X10 -4)
250
500 0
250
500
0
250
500
165
0
5
10 0
5
10 0
5
10
0
5
10
0
15
30 0
15
30 0
15
30
0
15
30
Age (ky)
165
170
170
175
175
180
180
0
10 0
5
5
10 0
5
10
0
5
10
TOC-MAR (g/cm2/ky)
TOC (%)
Fig. 4. Comparison of TOC and Ba/Al vs. TOC-MAR and Ba bio MAR for S6 on all studied sites.
shade
to visible sapropel
layer.
Light
sapropel.
Fig. 4. Comparison ofDark
TOC
and corresponds
Ba/Al vs. TOC-MAR
and Babio
MAR
forshade
S6 onrepresents
all studiedoxydized
sites. Dark
shade corresponds to visible sapropel
layer. Light shade represents oxydized sapropel.
964
Ba/Al (X10
0
969
966
964
200
400 0
969
966
Babio MAR
(ug/cm2/ky)
-4)
200
400 0
200
400
0
10
20 0
10
20 0
10
20
15
30 0
15
30
190
190
195
195
Age (ky)
934
35
200
200
205
205
0
5
10 0
5
10 0
5
10
TOC (%)
0
15
30 0
TOC-MAR (g/cm2/ky)
Fig. 5. Comparison of TOC and Ba/Al vs. TOC-MAR and Ba bio MAR for S7 on three studied sites.
Fig. 5. Comparison
ofshade
TOC and
Ba/Al vs. TOC-MAR
Babio MAR
forLight
S7 onshade
three studied
sites. oxydized
Dark shadesapropel.
corresponds to visible
Dark
corresponds
to visibleand
sapropel
layer.
represents
sapropel layer. Light
shade represents
oxydized sapropel.
Maximum
Maximum
values off-scale
are shown
on the values
graph.off-scale are shown on the graph.
intersection essentially at the origin of the y axis. Although
this intersection point is not exactly zero, the amount of inorganic N is negligible, as C:N ratios of low TOC samples
are within the range of previous studies (e.g., Meyers and
Arnaboldi, 2005) and in agreement with expected Redfield
ratios.
Because organic N is also susceptible to post-depositional
oxidation similar to organic carbon, its use for paleoproductivity reconstruction is subject to the same restrictions.
Nonetheless, the correlation between TN and TOC gives us
confidence that the entire N in the sediment is associated with
or derived from organic matter, and so our δ 15 N analyses will
reflect purely Norg .
Biogeosciences, 8, 415–431, 2011
4.1.3 δ 15 N patterns in sapropel sequences
The Norg isotopic composition can be used as an indicator for
the origin of organic matter (e.g., Knicker and Hatcher, 2001;
Meyers, 1997; Rinna et al., 2002; Schubert and Calvert,
2001) and, more importantly, for evaluating nutrient cycles
in the water column (e.g., Altabet et al., 1999; Altabet and
Francois, 1994; Freudenthal et al., 2001; Karl et al., 2002;
Pantoja et al., 2002; Voss et al., 1996). The Mediterranean is
presently an oligotrophic sea, characterized by low concentrations of macro-nutrients such as P and N in surface waters (e.g., Astraldi et al., 2002; Bethoux, 1989; Struck et al.,
2001). This shortage of nutrients, as a whole and of P specifically, is particularly dramatic in the Eastern Basin. An important regional nutrient source to the Eastern Mediterranean
is the Nile River (e.g., Diester-Haass et al., 1998; Jenkins
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D. Gallego-Torres et al.: Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels
Babio MAR
(ug/cm2/ky)
d15N (‰)
TOC (%)
0
7.5
15
0
10
0
20
40
-26
-23.5
-21
0,0
15
0
5
10
0
10
20
-5
2,5
60
-24
-21.5
-19
0,0
15
30
0
15
30
-5
3
10
-26
20
-5
2.5
10
1730
1735
S30
1740
30
10
1800
1805
S32
1810
Age (kys)
1815
0
30
0
10
0
50
100
-23
-20
0,0
12.5
0
10
20
0
15
30
-5
2.5
0
50
100 -26
-23
-20
0,0
20
2940
S52
2945
25
10
2965
S53
2970
TOC-MAR
(g/cm2/ky)
d13C (‰)
12.5
25
C:N (atm ratio)
Fig. 6. Geochemical signal and TOC and Babio Mass
Fig. 6. Accumulation
Geochemical signal
andC:N,
TOCand
and CBa-bio
Mass AccumulaRates,
N isotopic
tion Rates,
C:N,
and
C-N
isotopic
composition
for
representative
composition for representative Lower Pleistocene
Lower Pleistocene
fromand
SitePliocene
967 and Pliocene
sapropels
from964.
Site
from Site 967
sapropels
from Site
964. Note
different
scales
for TOC
and Ba-MAR’s.
Shaded
areas
Note
different
scales
for TOC
and Ba-MAR's.
Shaded
correspond
to sapropel
layers.to sapropel layers.
areas
correspond
www.biogeosciences.net/8/415/2011/
421
and Williams, 1984; Rossignol-Strick, 1985; Weldeab et al.,
2003a). This riverine input of nutrients has fluctuated over
time as cyclic intensification in the monsoon system during
insolation maxima increased runoff from its catchment (e.g.,
Hilgen, 1991; Rohling and Hilgen, 1991; Rossignol-Strick,
1985). With respect to N, nitrogen fixation and possibly
atmospheric deposition are currently important sources for
bioavailable N to the oligotrophic Mediterranean, although
their significance is also variable over space and time.
Organic matter in all the sapropels exhibits a shift to δ 15 N
values lower than in the background marls at the four locations (Figs. 6 to 11). This signature is unlikely a result
of diagenesis as under suboxic-anoxic water column conditions the expected trend of organic matter degradation is an
increase in δ 15 N due to remineralization that is associated
with an increase in TOC/TN ratios (e.g., Karl et al., 2002;
Lehmann et al., 2002; Meyers and Arnaboldi, 2005; Nakatsuka et al., 1997). Moreover, Higgins et al. (2010) recently
rejected the possible diagenetic overprinting by measuring
chlorin δ 15 N in sapropels and marls. Instead, the possible
explanations for the dramatic decrease in 15 δN values in the
sapropels are: (a) incomplete nutrient utilization under nutrient excess conditions (e.g., Calvert et al., 1992), (b) an external source of light N such as river discharge or atmospheric
deposition (e.g., Krom et al., 2004; Mara et al., 2009), and
(c) an ecosystem change in which a bloom of nitrogen-fixing
primary producers occurs (e.g., Arnaboldi and Meyers, 2006;
Milder et al., 1999; Pantoja et al., 2002; Sachs and Repeta,
1999; Struck et al., 2001).
The evidence for increased productivity based on the high
TOC and Ba concentrations of the sapropels is strong so we
may reject the hypothesis of incomplete nitrogen utilization.
Fluvial delivery is also unlikely as an extraordinary excess of
nutrients would have to be delivered to lower the δ 15 N values, particularly considering the amount of organic carbon
accumulated in the sapropels (Sachs and Repeta, 1999). Increase in atmospheric deposition of nitrogen is also unlikely
as it is expected that less dust would be mobilized during the
wetter climate associated with precessional minima. In addition much of the atmospheric nitrogen sources today are
anthropogenic and thus atmospheric deposition was not as
important in pre-anthropogenic times. Accordingly we believe that nitrogen fixation is the main source of the bioavailable nitrogen with low isotope values in surface waters (e.g.,
Altabet and Francois, 1994; Kuypers et al., 2004; Milder
et al., 1999). This process would incorporate dissolved atmospheric dinitrogen (δ 15 N = 0‰) into the marine system,
lowering the isotopic composition (e.g., Altabet et al., 1999;
Karl et al., 2002; Meyers, 1997; Pantoja et al., 2002). The
extremely light N isotopic composition suggests a change
in the photoautotrophic community to a primarily N-fixing
biota, probably associated with a decrease in eukaryote phytoplankton activity during sapropel deposition times (e.g.,
Milder et al., 1999; Pantoja et al., 2002; Sachs and Repeta,
1999). Nitrogen fixing cyanobacteria, cyanobacterial mats
Biogeosciences, 8, 415–431, 2011
422
D. Gallego-Torres et al.: Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels
964
TOC (%)
0
2
4 -2
969
d15N
3
8
0
2
4 0
967
4
8
0
2
4
0
966
4
8
0
0
0
0
5
5
5
5
10
10
10
10
2
4 0
4
8
Age (ky)
0
15
15
-25
-23
-21 0
d13C
7
15
15
-25
-23
-21 5
10
15
15
-25
-23
-21
5
10
15
-22
-20
-18 5
10
15
C:N
Fig. 7. TOC, C and N ratio, d 15N, d 13C for S1 at all studied sites.
Shaded area corresponds
to organic enriched sediments
Fig. 7. TOC, C and
N ratio, δ 15 N, δ 13 C for
S1 at all studied sites. Shaded area corresponds to organic enriched sediments.
964
0
d15N
3
TOC (%)
5
10 -2
969
8
0
5
966
10 -2
3
0
8
115
115
120
120
120
125
125
125
130
130
5
10
-2
3
8
Age (ky)
115
20
-25
-23
d13C
-21 0
10
20
-25
-23
-21
5
12.5
130
-24
-22
-20
5
12.5
20
C:N
Fig. 8. TOC, C and N ratio, d 15N, d 13C for S5 at three studied sites.
Shaded area corresponds to organic enriched sediments
Fig. 8. TOC, C and N ratio, δ 15 N, δ 13 C for S5 at three studied sites. Shaded area corresponds to organic enriched sediments.
and/or diatom-hosted cyanobateria, (see review by Karl et
al. (2002) and Kemp et al., (1999)), produce organic matter that is very low isotopically in nitrogen. This phenomenon would favor low δ 15 N values, and at the same time,
it would provide extra bioavailable nitrogen for other primary
producers.
Nitrogen fixation is typically limited by Fe, Mo and/or
P availability (Karl et al., 2002). However, although river
runoff would not be able to supply enough N to support
the observed increase in productivity, it may have been capable of supplying oligoelements, such as Fe, in quantities
sufficient to induce higher productivity and N-fixation by
cyanobacteria and archaea. The extra freshwater input occurring across the basin (e.g., Gallego-Torres et al., 2010;
Osborne et al., 2010) decreased salinity in the upper part of
the water column. Lower salinity enhances Mo reactivity
and availability, in the form of MoO−2
4 , whereas P can readily be recycled by efficient scavenging in the water column
(Karl et al., 2002) or reductive dissolution at the sedimentwater interface (e.g., Slomp et al., 2004). These multiple
factors, along with an increase in sea surface temperature
Biogeosciences, 8, 415–431, 2011
(e.g., Emeis et al., 2000b; Lourens et al., 1992), probably
allowed the maintenance of blooms of N-fixing organisms
and associated biota.
The shift towards low δ 15 N values during sapropel events
is evident across the whole region and parallels or precedes the increase in export production indicated by Babio
MARs (e.g., Dehairs et al., 1987; Gallego-Torres et al., 2007;
Weldeab et al., 2003a). δ 15 N values exhibit a progressive
decrease from the onset of sapropel formation upwards, although not always parallel to Ba/Al enrichments, and generally preceding the productivity maxima, indicating that maximum N fixation was followed by an increase in total export
production. As a whole, low δ 15 N values coincide well with
increased organic carbon concentrations, in agreement with
other sapropel studies (e.g., Calvert et al., 1992; Meyers and
Arnaboldi, 2005; Milder et al., 1999; Struck et al., 2001). We
thus infer that blooms of nitrogen-fixing biota were a central
factor in creating the higher surface productivity that led to
sapropel formation.
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D. Gallego-Torres et al.: Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels
964
TOC (%)
0
3
6
969
d15N
-2
2
6
0
3
6
967
-2
2
0
6
3
6
423
966
-2
2
0
6
3
6
-2
2
6
165
Age (ky)
170
175
180
-25
-23
-21
5
12,5
d13C
20
-24
-22,5
-21
5
12,5
20
-24
-21,5
-19
5
15
25
-23
-21.5
-20
5
12.5
20
C:N
15
13
Fig. 9. TOC,
C and13
d N, d C for S6 at all studied sites.
Fig. 9. TOC, C and N
ratio,area
δ 15corresponds
N, δ NCratio,
forto S6
at allenriched
studiedsediments
sites. Shaded area corresponds to organic enriched sediments.
Shaded
organic
964
TOC (%)
0
4
8
969
d15N
-2
2
6
0
4
8
-2
2
966
6
0
190
190
195
195
195
200
200
8
-2
2
6
Age (ky)
190
4
200
205
205
-24
-22.5
-21
5
d13C
12.5
20
205
-24
-22.5
-21
5
12.5
20
-23
-21.5
-20
5
12.5
20
C:N
13 C
10. TOC,
Cδ
and
N for
ratio,
d 13C studied
for S7 at sites.
three studied
Fig. 10. TOC, C and NFig.
ratio,
δ 15 N,
S7d15atN,three
Shadedsites.
area corresponds to organic enriched sediments.
Shaded area corresponds to organic enriched sediments
4.1.4
Paleoproductivity patterns based on Ba and
Corg concentration.
Greater TOC concentration in sapropel sequences is consistently linked to an increase in Babio . Although Babio cannot be used to quantitatively determine export productivity
(e.g., Averyt and Paytan, 2004; Gingele and Dahmke, 1994;
Kasten et al., 2001; McManus et al., 1998, 1994), it has
been widely and successfully applied as a relative indicator for enhanced marine productivity in the Mediterranean
(e.g., Dehairs et al., 1987; Diester-Haass et al., 1998; Emeis
et al., 2000a; Weldeab et al., 2003a). Furthermore, it has
been shown that Ba content generally permits a better reconstruction of the duration of enhanced productivity and
thus, true sapropel duration, than TOC, because barite is less
sensitive to postdepositional oxidative destruction (e.g., Paytan and Kastner, 1996; Thomson et al., 1995). In Mediterranean sapropels, TOC destruction is evidenced by oxidation fronts in the form of high Fe and Mn concentrations
above sapropel layers coinciding with low TOC concentrawww.biogeosciences.net/8/415/2011/
tions, whereas high Ba concentrations typically remain preserved (e.g., Gallego-Torres et al., 2010, 2007; Larrasoaña et
al., 2003b; Martı́nez-Ruiz et al., 2000; Thomson et al., 1995,
1999).
TOC and Babio MAR’s are calculated assuming constant
LSR’s for each i-cycle (see Table 1). This assumption can
conflict with the accepted “isochronous sapropel model”
(e.g., de Lange et al., 2008; Rohling et al., 2006). However,
assuming isochronous sapropel boundaries implies shifts in
LSR’s of up to 90% higher or up to 45% lower from background to sapropel sediments. Nijenhuis and de Lange
(2000) proved the inconsistency of such shifts and suggested
that MAR’s should be calculated based on approximately linear sedimentation rate throughout each i-cycle. Owing to the
necessity of selecting one hypothesis, constant LSR’s were
assumed.
Although the amounts of Babio increase may vary among
the layers, they always accompany the shift to higher TOCMAR at the base of the sapropels. As a whole, the Late
Pleistocene sapropels, ranging in age from 81 ky to 217 ky
Biogeosciences, 8, 415–431, 2011
424
D. Gallego-Torres et al.: Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels
Fig. 11. Temporal evolution of the depositional conditions from Pliocene to Pleistocene, represented by TOC, Babio -MAR, N and C isotopic
composition and C:N ratio on the Mediterranean Ridge (Site 969). For Holocene sapropel, see Figs. 2 and 6. Maximum off-scale values are
numerically indicated in the graphs.
(see supplementary table) exhibit the highest export production levels of the studied sections, as represented by the highest Babio -MAR, although TOC and TOC-MAR’s are higher
during the Pliocene and Early Pleistocene. Thus, there is
a poor relation between TOC accumulation and productivity (Babio ), as presented on Fig. 13 (see also TOC/Babio on
supplementary table), which is particularly evident comparing Pliocene and Late Pleistocene layers. Factors other than
export productivity appear to have influenced sapropel formation during the Pliocene.
Biogeosciences, 8, 415–431, 2011
A disconnect between concentrations of Ba (unaltered signal for increased productivity) and TOC at the top of the
Holocene S1 sapropel across the basin (different gray shadings, Fig. 2) and, to a lesser extent, in some Late Pleistocene
sapropels, (Figs. 3 to 5; see also Gallego-Torres et al., 2010),
indicates post-depositional oxidation of the organic matter
at the top of the sapropel (e.g., Martı́nez-Ruiz et al., 2000;
Thomson et al., 1995; Van Santvoort et al., 1996) probably due to relatively easier and faster reventilation of bottom waters. Opposite to this trend, the Early Pleistocene and
Pliocene sapropels (Figs. 6 and 11) generally lack oxidation
www.biogeosciences.net/8/415/2011/
D. Gallego-Torres et al.: Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels
425
1.8
Ntot (%)
1.5
1.2
0.9
2
964
R = 0.9921
969
R = 0.982
966
R = 0.9898
a= 0.0531
2
a= 0.0476
2
a= 0.0623
0.6
0.3
0.0
0.0
5.0
10.0
15.0
20.0
25.0
30.0
TOC (%)
0.60
Ntot (%)
0.50
0.40
0.30
967
2
R = 0.9456
a= 0.0451
0.20
0.10
0.00
0.0
2.0
4.0
6.0
8.0
10.0
TOC (%)
Fig. 12. Total Nitrogen to TOC correlation.
Very high correlation indicates
that we may consider Ntot=Norg.
fronts. In fact, a reversed offset between TOC and Ba/Al
is
4.2 Sapropel expression across the eastern basin
Fig. 12. Total Nitrogen to TOC correlation. Very high correlation indicates that we may consider Ntot =Norg .
evident in i-cycles 282 and 284 (S52 and S53, at Site 964
see Figs. 6 and 11). These sapropels originally developed
as a result of an increase in export production recorded by
Babio although productivity was less than in some of the Late
Pleistocene sapropels. However TOC was preserved, reaching maximum concentrations continuing to accumulate after
the return to normal surface productivity, probably due to the
presence of anoxic bottom waters that favored its preservation. Anoxic conditions in the sediments could have remobilized Ba from the upper part of sapropels S53- (i-cycle 284)
and S28 (i-cycle 156) (Fig. 11) (e.g., Arndt et al., 2009).
The difference between the two paleoproductivity proxies
suggests that deep-water circulation in the eastern Mediterranean during the middle Pliocene was slower than in the late
Quaternary, leading to diminished deep-water reventilation.
From these lines of evidences we conclude that Pliocene
and Early Pleistocene sapropel formation resulted from increased TOC accumulation as a combined consequence of
enhanced productivity and better preservation despite relatively low sedimentation rates because of slower deep circulation and bottom-water oxygen depletion. In contrast lat
Quaternary sapropels developed under higher sedimentation
rates that mainly reflect higher productivity. Sluggish circulation during precessional minima also favored TOC accumulation, but as soon as productivity returned to normal
low values deepwater oxygenation partially degraded organic
matter in the upper part of sapropel layers.
www.biogeosciences.net/8/415/2011/
through time
Our results reveal basin-wide patterns, temporal variations,
and spatial differences in the history of sapropel deposition in the eastern Mediterranean Sea over the past 3 My.
The general trend towards lighter N-isotopic composition
in the sapropel layers is clear but not particularly strong
during the deposition of the Holocene S1 (i-cycle 2). The
most plausible explanation for this weak signal is a minor increase in nitrogen-fixing cyanobacteria population and
in overall marine productivity. The remaining Pleistocene
and Pliocene sapropels show similar and essentially concordant trends across the basin: a progressive decrease of δ 15 N
values from the onset of sapropel formation upwards, frequently preceding Ba/Al enrichment. At the oxidized top of
the sapropel layers, loss of material does not permit us to
assess whether N-fixation continued during the whole high
productivity phase.
The Ba-MARs during sapropel deposition are equivalent
for deep water sites (964, 969, and 967) whereas it is systematically lower at Site 966. The same pattern is seen in
TOC-MARs. Sites 966 and 967 are geographically proximate to each other, but their water depths are very different
(926 m.b.s.l. vs. 2555 m.b.s.l). Export production, which is
a surface ocean process, is expected to be similar at both
sites. The difference in Ba MARs may be instead related
to the degree of barite saturation and thus its preservation
in the water column, which is depth dependent (e.g., Paytan
and Griffith, 2007; Van Beek et al., 2006). In any case, for
Biogeosciences, 8, 415–431, 2011
426
D. Gallego-Torres et al.: Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels
90
30
60
20
969
967
Babio MAR
TOC MAR
964
30
966
10
964
969
967
966
0
0
15
0
30
TOC (%)
0
50
TOC MAR
100
Fig. 13. Correlation between TOC vs. TOC-MAR and between TOC vs. BabioMAR
Sites 964,
967. Maximun
TOC values
with highest
Fig. 13. Correlation for
between
TOC969
vs.and
TOC-MAR
and between
TOCdo
vs.not
Bacorrespond
bio MAR for Sites 964, 969 and 967. Maximun TOC values do
and
viceversa.
bioMAR
not correspond with Ba
highest
Babio
MAR
and viceversa.
-5.0
-5.0
-10.0
d13C
C4 land
plants
-15.0
-20.0
-15.0
Marine
cyanobacteria
Marine
algae
-20.0
-25.0
-30.0
0.0
5.0
10.0
15.0
20.0
Site 964-sapropels
Site 964-background
Site 966-sapropels
Site 966-background
-25.0
C3 land
plants
Lacustrine
algae
25.0
8.0
6.0
4.0
2.0
0.0
-30.0
-2.0 -4.0
d15N
C:N
-5.0
-5.0
-10.0
-10.0
C4 land
plants
d13C
-10.0
-15.0
-15.0
Marine
algae
-20.0
Site 967-sapropels
Site 967-background
Site 969-sapropel
Site 969-background
-20.0
-25.0
-25.0
C3 land
plants
Lacustrine
algae
-30.0
0.0
5.0
10.0
15.0
20.0
25.0
30.0
C:N
8.0
6.0
4.0
2.0
0.0
-30.0
-2.0 -4.0
d15N
Fig. 14. Origin of the organic matter contained in the sampled sapropels
15
13
Fig. 14. Origin of the organic
matter contained
the
as a function
of d15N,ind13
C sampled
and C:Nsapropels
ratios. as a function of δ N, δ C and C:N ratios.
the same insolation cycle, the relatively shallow Site 966 exhibits the lowest TOC and Ba MARs of the four sites (Figs. 2
to 5). Therefore, it appears that greater water depth favors
the burial and preservation of organic matter as proposed by
Murat and Got (2000) and refined by de Lange et al. (2008),
although it is not a factor influencing the primary process
of sapropel formation itself. Sapropels on the top of Eratosthenes Seamount, although possessing lower TOC values, are
characterized by all of the same features as those in the deep
basin (increase in Ba/Al and Ba-MAR, lower δ 15 N, higher
TOC/TN). These proxies point toward sapropel formation
being induced by basin-wide increased surface productivity,
at least during the period between i-cycles 20 and 2, where
we are able to compare similar layers across the basin.
In terms of time evolution of sapropel events, productivity maxima were reached during i-cycle 12 (S5), coinciding with maximum monsoon index (Kraal et al., 2010;
Rossignol-Strick and Paterne, 1999). This pattern allows us
to conclude that the same climatic factors (warm/wet periods during summer insolation maxima and enhanced monsoon activity) that triggered increased fluvial discharge and
Biogeosciences, 8, 415–431, 2011
amplified nutrient input also favored the establishment of Nfixing bacteria. This combination of factors ultimately induced sapropel formation.
From Figs. 3, 4 and 5, we observe that the duration of
some sapropels (e.g., S5 to S7) appears to become longer
from west to east. This pattern may indicate either a progressively shorter increased productivity event towards the west
or an increase in sedimentation rates associated with sapropel forming conditions. A similar problem was addressed
by Nijenhuis and de Lange (2000) and recently Katsouras et
al. (2010) showed that S1 developed later in the Aegean Sea
compared to the Lybian coast, by an offset of up to 0.9 ky.
We do not have sufficient information (particularly, absolute
dating) to resolve this issue. However, we are inclined to believe that this pattern results from differences in sedimentation rates inasmuch as the climate-driven surface paleoceanographic conditions that favored increased productivity were
approximately stable and concordant across the basin.
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D. Gallego-Torres et al.: Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels
4.3
Diagenesis
The combination of TOC/TN ratios and δ 13 C and δ 15 N values has been used to determine the origin of the organic
matter in marine sediments (e.g., Bouloubassi et al., 1999;
Meyers, 1997; Oldenburg et al., 2000; Rullkötter, 2000) by
distinguishing between contributions from marine algae, marine cyanobacteria, lacustrine algae, and continental C3 -C4
plants, as summarized by Meyers (1997). These parameters
are plotted in Fig. 14 for the 35 analyzed sapropels. Holocene
sediments plot close to the marine algae end member, with
minor deviations coinciding with the highest TOC values.
Pleistocene and Pliocene samples show a nearly bimodal distribution on the TOC/TN-δ 13 C plot. The great majority of
low TOC samples fall in the marine domain, the exception
being a few samples from Site 967. Samples from within
the sapropel shift towards higher C:N fields. In contrast,
the δ 15 N vs. δ 13 C plot systematically shows a marine algae
composition for non-sapropel samples and a typical marine
cyanobacteria composition for sapropel samples.
Although the TOC/TN vs. δ 13 C relation (Fig. 14) could
be misinterpreted as resulting from an influence of continental input (i.e., C3 plants), the low δ 15 N and δ 15 N vs. δ 13 C
plots suggest that the sapropels are dominated by marine organic matter, and independent evidence such as large values
of the Rock-Eval Hydrogen Index (Bouloubassi et al., 1999;
Emeis et al., 1996) and an abundance of marine biomarker
molecules (Rinna et al., 2002) supports this interpretation.
Nitrogen isotopic compositions are only slightly or not at all
affected in preserved organic matter (Higgins et al., 2010),
whereas under conditions of high surface productivity, the
typical TOC/TN value of the exported organic matter is
higher than the typical algal signal, partly due to selective
remineralization of N and N-rich molecules below the euphotic zone and/or at the sediment surface (e.g., Twichell et
al., 2002). According to Freudenthal et al. (2001; see also
Macko et al., 1994), TOC/TN ratios would increase when
remineralization and preferential degradation of amino acids
(isotopically higher) occurs along the water column. Similar results were obtained in sediment traps studies done by
Van Mooy et al. (2002). Selective remineralization of amino
acids simultaneously yields lower δ 15 N values and higher
TOC/TN values of deposited organic matter. The increase
in TOC/TN can also partly be caused by denitrification under a suboxic environment (Karl et al., 2002) that is easily achieved under highly productive waters. These diagenetic changes support the conclusions made by Arnaboldi
and Meyers (2006) on similar sapropel-bearing sediments in
the eastern Mediterranean. It is evident that this increase in
TOC/TN ratio is more extreme during the Pliocene, in agreement with a highly restricted water column oxygenation for
this period as postulated by Passier et al. (1999), Böttcher et
al. (2003), Warning and Brumsack (2000), Larrasoaña et al.,
(2003a), and Weldeab et al. (2003a, b).
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427
Lower N-isotopic composition of organic matter mirrors
the organic-rich levels, but it does not extend above oxidized
sapropels to parallel the increase in productivity. Because
nitrogen fixation seems to be crucial for sapropel formation,
changes in export productivity (Babio ) should run parallel to
N-fixing activity (δ 15 N), inasmuch as both reflect primary
production. However, the post burial burn-down oxidation
may affect this signal. Specifically, oxidation of deposited
organic matter preferentially consumes 14 N, so that the remaining organic matter in the sediment becomes enriched in
15 N thus altering the δ 15 N signal. Higgins et al. (2010) postulate that the N isotope signal is not affected by oxidation
of decaying organic matter, although the resolution of their
analyses is not high enough to discriminate oxidation fronts.
However, if we accept their hypothesis, we may infer that the
increase of δ 15 N values immediately above sapropels S1 or
S7 (light shading in Figs. 2 and 5) is due to better oxygenation of the overlying water column while high productivity
conditions still prevailed.
5
Summary and conclusions
The remarkably lighter N isotopic compositions of the sapropel layers indicate periods of nitrogen fixation and thus, amplified cyanobacterial productivity (probably associated with
bacterial mats) in the water column. Although N-fixation is
frequently associated with oligotrophic basins, in the case
of Eastern Mediterranean sapropels it created a highly productive environment. This increase in N-fixing biota and
bacterial primary productivity increased export production
and is reflected in major increases in TOC and Babio MARs.
The change in mode of productivity is fueled by nutrient input (such as P, Fe or Mo), most likely through riverine discharge and by a warming environment with higher SST and
lower SSS (Emeis and Weissert, 2009; Emeis et al., 2000b).
These climatically driven productivity events affected the
entire Eastern Mediterranean, independent of water depth.
Differences in sapropel expression depend on water depth,
related to winnowing, deep water circulation and/or SO−2
4
saturation.
Our results allow us to distinguish between the importance
of organic matter production and its preservation in forming
the sapropel layers. TOC-MAR and Babio MAR do not show
a direct correlation with Corg concentration and Ba/Al ratio,
respectively. Some Late Pleistocene sapropels, such as S5
(i-cycle 12) have lower TOC concentrations but higher TOC
and Babio MAR’s than older equivalents, indicating that both
marine productivity and detrital sedimentation dramatically
increased during the Late Pleistocene. Opposite to this trend,
Early Pleistocene and Pliocene sapropels accumulated under markedly lower sedimentation rates and therefore exhibit
higher Ba concentrations but lower MARs than Late Pleistocene equivalents. In other words, greater productivity does
not directly result in higher TOC accumulation (Fig. 13). The
Pliocene and Early Pleistocene relatively smaller increases
Biogeosciences, 8, 415–431, 2011
428
D. Gallego-Torres et al.: Productivity patterns and N-fixation associated with Pliocene-Holocene sapropels
in productivity correspond to limited water mass circulation
than in Late Pleistocene times; water mass circulation may
have even reversed (estuarine circulation, e.g., Böttcher et al.,
2003; Wehausen and Brumsack, 1999) favouring bottom water stagnation. This interpretation implies that organic carbon
concentration in these layers is equally influenced by both
preservation and productivity. Variations in deep water circulation are also responsible for different diagenetic imprints
on TOC, TOC/TN ratio and δ 15 N signals.
Humidity maxima also contribute to the intensity of productivity increase by magnifying fluvial nutrient input into
the basin. Higher humidity signifies stronger river runoff,
higher nutrient supply from the continent and, at the same
time, increased detrital input that would increase sedimentation rates. If we consider the increase in African derived
nutrient input as climatically controlled (in turn defined by
insolation cycles), the maximum increase in fluvial input
should correspond to i-cycles 12 to 20, which show the greatest maximum summer insolation (500 to 520 W/m2 ) according to Emeis et al. (1996) and Lourens et al. (2004). Sampled sapropels from i-cycles 152 to 182 correspond to insolation peaks that remain around 500 W m−2 (Larrasoaña et
al., 2003a) or below, for i-cycles 270 to 286. Thus, detrital
and nutrient input would be the highest at maximum insolation (i.e., Late Pleistocene). This prediction is supported by
our results that reveal relatively low TOC concentration compared to older equivalents and, at the same time, the very high
TOC and Babio -MARs found at all four sites, for example
during i-cycle 12. The relative sizes of the increases in sedimentation rates and productivity rates are evidently related
to the amplitudes of the precessional insolation and humidity
maxima (monsoon index).
A remarkable increase in TOC/TN ratios, related to denitrification processes, suggests intense oxygen consumption
below the euphotic zone. The δ 15 N signal, although mostly
representing a primary signal, might be shifted to somewhat
larger values by diagenetic oxidation that is evident in diminished TOC concentrations at the tops of sapropel layers
because of oxidative burndown.
Supplementary material related to this
article is available online at: http://www.biogeosciences.
net/8/415/2011/bg-8-415-2011-supplement.pdf.
Acknowledgements. We are grateful to the Ocean Drilling Program
for providing us with the samples used in this study as well as to
the ODP Core Repository (Bremen, Germany) for assistance with
sampling. Major and trace elements analyses were performed at
the “Centro de Intrumentacion Cientifica” (University of Granada).
TC-TOC measurements were done at the Stable Isotope Lab
(Green Earth Science Building, Stanford University) and Bremen
University (MARUM). Stable Isotope analyses were done at Stable
Isotope Lab (Green Earth Science Building, Stanford University.
We also acknowledge the collaboration of Dr. Oscar Romero in
TC-TOC analysis at Bremen University. We thank R. Capozzi,
Biogeosciences, 8, 415–431, 2011
K. C. Emeis and one anonymous reviewer, for their comments
that were very helpful in improving this manuscript, as well as
A. Shemesh for his Editor labour. This work was financed by
Projects MARM 200800050084447, MICINN CGL2009-07603,
Junta de Andalucı́a RNM-5212 and Research Group RNM 179
(Junta de Andalucı́a), We also thank Project CSD2006-00041
Topoberia. We would also like to thank the “Plan Propio de Investigación (UGR)” for financial support (postdoctoral fellowship
D. Gallego-Torres).
Edited by: A. Shemesh
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